This invention relates to systems for amplifying optical signals and, more particularly, to a system and method for linearly amplifying optical analog signals by backward stimulated Raman scattering.
There are instances where weak electromagnetic energy, especially optical signals, must be amplified after the signals have traveled a long distance. Such application may include the detection and measurement of optical signals which are generated in a deep hole or a tunnel. These optical signals may be generated, for example, by an underground explosion. One of such optical signals may be the intensity of radiation of chemical compounds.
A conventional technique is to use an opto-electrical transducer for transforming such optical signals into the appropriate electrical signals. The electrical signals are then transmitted on a conventional metallic coaxial cable the other end of which is connected to a conventional detector. Such a technique has several disadvantages. One disadvantage is that such a metallic cable has an inherent characteristic of eliminating the high frequency components of a signal, resulting in the distortion of the time duration of the signal after it has traveled through the entire length of the coaxial cable, e.g., one kilometer. An analog electrical signal having a time duration of one nanosecond could be stretched into a signal of a few tens of nanoseconds; a signal having a time duration of one nanosecond is generally stretched to three nanoseconds after travelling approximatel 300 feet. In addition, the amplitude of the signal will also be lost after travelling such a long distance. Moreover, metallic cables tend to add noise to the desired electrical signal. In addition to the fact that metallic cables are expensive and heavy, it is capable of conducting lightning into the underground test site, causing damage to other equipment.
A second technique in detecting analog optical signals is to use optic fibers in conjunction with equipment such as spectral equalizers. One inherent disadvantage of such an optic fiber is its propensity to stretch out the time duration of the broad-spectrum optical signal. In addition, the optical signals generated by the underground explosion invariably have insufficient intensity such that detecting that intensity at one frequency is frequently impossible. Spectral equalizers are therefore used to compensate for the lack of intensity. A conventional spectral equalizer utilizes 10 fibers each of which is conducting a particular frequency of the generated optical signal. The optical signal is first grated into ten frequencies before each of the frequencies is fed into a fiber. Each of the fibers has a different length so as to compensate for the velocity of each frequency such that all frequencies of the optical signal arrive at the detector of the spectral equalizer at the same time. The intensities of all the frequencies are then accumulated such that the combined intensity can be detected. The combined intensity, however, is not a true amplification of the optical signal, but rather, an accumulation of the intensities of that signal. This technique is capable of increasing the intensity by approximately three times. Since the spectral equalizer is only capable of slightly increasing the intensity of the optical signal, informational contents of the optical signal are frequently lost. For example, if the optical signal is the intensity of radiation of a chemical compound, that signal contains spectral information that could be deciphered by spectroscopic equipment. Another disadvantage in using spectral analyzers is that such equipment requires extensive calibration and manpower support.
A third technique is to digitize the detected optical signal. In such a technique, the presence of such a digital optical signal represents the occurrence of an optical event. It, however, is incapable of presenting other informational contents of the optical signal such as the fast-varying, detailed spectral data or the time profile of that signal when that information is desired. In amplifying such a digital optical signal, the signal is transmitted in a conventional optic fiber the other end of which is connected to a laser source. In conjunction with the emitted laser beam from the laser source, the optic fiber facilitates amplification of the digital optical signal by stimulated Raman scattering. Another disadvantage of such a technique is its inherent inability to digitize high frequency optical signals.